The present invention refers to a process for production of high density sintered parts from spherical metal powders, as well as to the sintered parts obtained by said process.
Powder metallurgical processes, such as sintering, allow the production of complicated details almost without subsequent machining and is therefore an advantageous method for making small and medium sized structural parts or components.
It is well known that spherical powders made from gas atomizing a liquid melt in an inert atmosphere yield a powder with high purity. If such a powder for example of a stainless steel or a nickel base alloy is densified at high temperature to full density, the result is an excellent product with properties in many cases superior to wrought products. There are some known techniques to produce such products for example Hot Isostatic Pressing (HIP) and Metal Injection Moulding (MIM). The first mentioned technique, HIP, where you use mainly metallic capsules to enclose the powder body, is mainly suitable for bigger objects for economical reasons. The powder has to be encapsulated before the pressing and after the pressing process machining will be required to remove the capsule which has been attached to the pressed powder material. The second process, MIM, is suitable for small objects, usually of a very intricate form, up to a maximum of 1 kg, but this process is rather expensive due to the requirement of finer powders and long process times. This process also requires that the powder is mixed with for instance a plastic material before extrusion. The plastic material has to be removed before sintering giving a green body of a low density. Due to the above mentioned reasons the MIM process therefore is mainly used when small parts of very complicated shape can replace extensive machining.
It is known that you can reach very interesting results in material working, like for example cropping of steel bars, cutting blanks in steel sheath etc., by using so called high speed forging presses. This technique was developed in the early 60""s even if the fundamentals were known since the 40""s, to be used to cut small diameter bar into blanks for further processing, like for example valve heads for engine valves in automobile industry which are produced in many millions around the world and which require a process with high production capacity as well as high yield in the cropping process.
With high speed presses for cropping or blanking we mean presses with ram speeds, that is tool speeds, over approximately 3-4 m/s and preferably higher. In this type of process, for example when cropping the above mentioned small bars, you get a phenomena called adiabatic heating, which can be explained as follows: When a material is deformed this is done in distinct planes and directions, so called shear planes or shear bands. In these planes a part of the deformation energy subjected to the body is transformed to heat energy. If now the deformation energy is applied to a massive body fast enough, the shear planes will momentarily by heated to a high temperature, so called adiabatic heating, and the material will be sheared off along this plane. If the deformation speed is too low, the heat conductivity will more and more reduce the effect of adiabatic heating resulting in cut off blanks with rugged surfaces and cracks etc., and with uneven tolerances. This is known technique.
In uniaxial powder pressing, you use traditionally slow going hydraulic presses where in a closed tool irregular powders are compressed into blanks which are then sintered to obtain better mechanical properties. For standard powders like carbon steel and stainless steel you never reach full density, so the end product is porous and the usage potential is therefore limited. However, uniaxial pressing is a very efficient production technique to make near net shape products and it would therefore be very interesting if you could find a method to reach full density products having the same properties as wrought products. In uniaxial pressing one of the limiting factors is the maximum surface pressure in the tools and this limit is usually practically in the area of 700-800 N/mm2. For these standard powders you get some improvement by heating the whole powder mass before introducing the powder mass into the tool, so called warm compaction process, which has been introduced during the last few years. You get some improvements in green density, but they are not very dramatic and especially on high alloyed material, like stainless steel, the effect is negligible.
Some tests of high speed pressing have been made with these types of irregular powders but no significant effect has been seen regarding improvement in density or mechanical properties. This is probably due to the fact that irregular powders due to their shape have a limited ability to shrink, as well as a high level of oxygen in the form of oxides on the surfaces, and other impurities.
Spherical metal powders produced by gas atomization suffer from low green strength after uniaxial compaction. In contrast the irregular metal powders produced by water atomization provide excellent green strength, but are heavily oxidized during production and said oxide films are hindering the subsequent sintering. U.S. Pat. No. 5,460,641 discloses a process for the preparation of an agglomerated metallic powder capable of sintering after cold compression forming, wherein spherically shaped metallic particles are mixed with an aqueous solution of gelatine to a pasty mixture which is granulated and dried. After cold compression of the agglome-rated metallic powder in a mould a green body is obtained having a mechanical strength which is superior to that obtained with the initial metallic particles and sufficient for handling and subsequent sintering. This process can be used for producing low density sintered parts from spherically shaped metallic particles.
The object of the present invention is therefore to provide a process that overcomes the drawbacks of the above mentioned methods giving high density green bodies with even better green strength, which subsequently can be sintered to high density metal parts.
Surprisingly it has now been found that fully or near fully dense products can be obtained from agglomerated spherical metal powders by means of high speed pressing and subsequent sintering. By using said process products of complicated shape can be obtained without the need for extensive machining. The products obtainable from the process according to the present invention have improved strength properties and can be used in very demanding environments.
The present invention refers to a process for compressing an agglomerated spherical metal powder comprising at least 0.5% by weight of a thermo-reversible hydrocolloid as a binder, which is characterized in that the agglomerated spherical metal powder is pressed in an uniaxial press operation with ram speed of over 2 m/s to a green body having a high density.
The present invention also refers to a process for the preparation of a sintered product from agglomerated spherical metal powder comprising at least 0.5% by weight of a thermo-reversible hydrocolloid as a binder, wherein the agglomerated spherical metal powder is pressed in an uniaxial press operation with a ram speed of over 2 m/s to a green body, and said green body is subsequently sintered to full or near full density.
The spherical metal powder to be compressed and optionally sintered is preferably a gas atomized metal powder, but can also be a spherical metal powder obtained in any other conventional way, such as by chemical or electrolytical precipitation. The metal powder can be a powder of a carbon steel or stainless steel, or any other high melting alloy based upon nickel, iron or cobalt. The alloy may also comprise other elements in smaller amounts, e.g. carbon, chromium, molybdenum, copper, nitrogen, vanadium, sulphur, titanium and niobium. Alloys based on tantalum or wolfram are, however, not suitable as having a too high melting point, about 3000xc2x0 C. The expression xe2x80x9cspherical metal powderxe2x80x9d as used in the this context refers in addition to spherical also to near spherical metal powders, for instance of an oval shape.
Thermo-reversible hydrocolloids refer to hydrophilic colloidal materials which are characterized by a heat reversible gelling and softening which can be controlled by cooling and heating, respectively. Specific examples of such thermo-reversible hydrocolloids are slightly esterfied pectins, xcexa-carrageenan and gelatines. Further examples of said hydrocolloids are described in the publication Hydrocolloides, edited by Mero Rousselot Satia, Paris. In order to agglomerate the metal powder the binder is preferably added to the spherical metal powder in the form of an aqueous solution. The amount of binder in the agglomerated powder should normally be higher than 0.5% by weight as the binding properties are not sufficient below 0.5%. The amount of binder in the agglomerated powder should not be too high as this might cause problems when the binder is removed. A preferred upper limit is 1.5% by weight. According to a preferred process gelatine is used as a binder. Further details of the agglomeration process using gelatine as a binder are described in the U.S. Pat. No. 5,460,641. The gelling process, which normally occurs between 40 an 80xc2x0 C. when using gelatine, should consequently be reversible unlimited times.
The uniaxial press operation is a process where you use a tool which preferably is closed, for compressing the agglomerated spherical-metal powder in one single direction. The tool should be operating with a ram speed over 2 m/s whereby a green body is formed. The ram speed is according to a preferred embodiment of the invention 4 m/s or higher, e.g. 4-7 m/s. The high ram speed gives a pre-sintering product, that is a green body, having a high green density. The upper limit for the ram speed is determined by the strength of the tool. When too high a speed is used, the tool will disintegrate and fall into pieces. The pressure is in general 400-800 N/mm2. When the binder is gelatine the uniaxial press operation is preferably performed at a temperature ranging from 40xc2x0 C. to 55xc2x0 C., most preferred from 45xc2x0 C. to 50xc2x0 C.
The sintering of the green body takes place at a sintering temperature, which depends on the composition of the metal powder, and in a controlled atmosphere. The optimum temperature can be determined by conventional means, for example by using a software called Thermo-calc. The sintering temperature for steel powders and powders of high melting alloys will in general be within the range of 1100-1350xc2x0 C. and 1350-1550xc2x0 C., respectively. A stainless steel can for instance be sintered at 1350xc2x0 C. for 2 to 3 hours. The sintering normally takes place in vacuum or in a reducing or inert gas, preferably in hydrogen. The sintering gives a final product with full or near full density. Before the sintering the binder is removed by preheating in air at a temperature of 300 to 500xc2x0 C.
According to a preferred embodiment, the sintered product is subsequently subjected to hot isostatic pressing (HIP) without being encapsulated, whereby a product,of a guaranteed 100% density can be achieved.
The invention also refers to a sintered product obtained from an agglomerated spherical metal powder comprising at least 0.5% by weight of a thermo-reversible hydrocolloid as a binder by pressing the agglomerated spherical metal powder in an uniaxial press operation with a ram speed of over 2 m/s to a green body, and subsequently sintering said green body to full or near full density. In accordance with preferred embodiments of the invention the ram speed of the press during the pressing operation is 4 m/s or-higher, the amount of binder in the agglomerated powder does not exceed 1.5% by weight, and the thermo-reversible hydrocolloid is gelatine.
The invention also refers to a sintered product as well as to a green body obtainable from the processes described above. The yield of the process is over 98%.
The sintered products of the invention can be used for production of high strength, non-oxidising, corrosion resistant or fire proof products. Examples of such products are filters, gear box parts, such as high torque gear box parts, engine parts, fasteners, watch cases, valve parts like gates, and other details.